1. Field of the Invention
The present invention relates to an image heating apparatus suitable for use as an image heating fixing apparatus mounted to an image forming apparatus such as an electrophotographic copying machine and an electrophotographic printer, and to a flexible sleeve used for the image heating apparatus.
2. Description of the Related Art
As an image heating fixing apparatus (fixing device) to be mounted to an image forming apparatus such as an electrophotographic copying machine or a printer, there exists a film heating type apparatus. The film heating type fixing apparatus includes a heater having a resistance heat generating member on a substrate made of a ceramic, a flexible fixing film which moves while being in contact with the heater, and a pressure roller which forms a nip portion together with the heater through the fixing film. Japanese Patent Application Laid-Open Nos. S63-313182, H02-157878, H04-44075, and H04-204980 describe this type of fixing apparatus. A recording material bearing an unfixed toner image is heated while being pinched and conveyed at the nip portion of the fixing apparatus. As a result, the toner image formed on the recording material is fixed onto the recording material by heating. This fixing apparatus has an advantage in a short time required for raising temperature to a fixable temperature after starting energizing the heater. Therefore, a printer to which the fixing apparatus is mounted can reduce a “first printout time (FPOT)” corresponding to a time period required for outputting a first image after input of a print command. The fixing apparatus has another advantage in its low power consumption during a standby time for a print command. Since the above-mentioned fixing apparatus has the advantages as described above, the fixing apparatus has been mounted to not only a low-speed image forming apparatus but also a high-speed image forming apparatus. When the fixing apparatus is mounted to the high-speed image forming apparatus, it is necessary to supply a sufficient amount of heat energy to the recording material passing through the nip portion for a shorter period of time. Thus, the use of a metal sleeve including a metal base layer made of stainless steel (SUS) excellent in thermal conductivity as a fixing film has been proposed.
Moreover, the fixing film must be provided with durability over a long period of time. In recent years, energy saving and space saving are stringently required. Therefore, a diameter and a thickness of the fixing film used for the image forming apparatus are progressively reduced. With such reduction, the metal sleeve is required not only to have sufficient wear resistance but also to be excellent in characteristics such as flex resistance and durability.
However, if the diameter of the flexible metal sleeve is reduced in the case where the flexible metal sleeve is used as the fixing film, because a curvature radius becomes small, a flexural strength decreases. Therefore, the thickness of the metal sleeve is required to be reduced so as to ensure the flex resistance. However, there is a problem in that a process of reducing the thickness of the metal sleeve is difficult.
For example, in a case of the flexible sleeve manufactured based on a stainless sleeve having an outer diameter of 30 mm, the durability can be ensured even when the flexible sleeve is mounted to the film heating type fixing apparatus if the thickness of the stainless sleeve is reduced to about 35 μm to 40 μm. However, when the outer diameter of the stainless sleeve is reduced to 18 mm to 24 mm, it is difficult to ensure the durability for long time use even when the thickness of the stainless sleeve is reduced to about 25 μm to 30 μm. With the current stainless sleeve manufacturing technique, the thickness cannot be reduced to less than about 25 μm. Therefore, it is technically difficult to mount the stainless sleeve having the outer diameter of 18 mm to 24 mm to the film heating type fixing apparatus.